Most blow out preventers used when drilling for oil and gas are manually activated. A member of the drilling crew presses a switch on the master control panel when readings indicate, by a pressure increase within the annulus, that "blow out" conditions may exist. A blow out is an uncontrolled release of fluids up the annulus of the oil or gas well. Under normal drilling conditions the pressure of oil and gas endeavouring to escape up the annulus is controlled by the weight of a column of drilling fluid. When the pressure of oil and gas endeavouring to escape up the annulus exceed a counter-pressure exerted by the weight of the column of fluid, a "blow out" occurs. Once activated, the blow out preventer closes the annulus with two large hydraulic rams.
Problems arise when the drilling crew is not attentive and miss warning signs that precede a blow out or when the drilling crew has no or no sufficient warning of an impending blow out.